1. Field of the Invention
This invention relates to automated storage libraries. More particularly, this invention relates to a magnetic picker mechanism for a gravity feed storage library.
2. Description of the Prior Art
Many business and technical applications require very large databases for storing information used in connection with the applications. Database storage requirements can exceed hundreds or thousands of gigabytes of data. Often these storage requirements can not be met by disk drive storage due to cost and/or physical space limitations. Data storage for such large databases are typically stored on magnetic tape which provides the lowest storage cost per unit of data.
Tape storage, however, generally requires the longest access time among existing technologies for retrieving the stored data. There are two primary reasons for this. First, once a tape is loaded into a tape drive, accessing the data stored on the tape is substantially slower than accessing data stored on devices such as a direct access storage device (DASD) or an optical disk. This is primarily due to the respective transport speeds of the data containing medium as well as the bandwidth attainable by the read heads present in the respective storage devices. Moreover, since tape is accessed sequentially, a substantial delay can occur prior to locating the required tape segment. Secondly, the time required to locate the desired tape and transport it to the tape drive for loading can bring about substantial delays.
Early magnetic tape databases required an operator, when prompted, to retrieve a tape from a shelf and load the tape into a tape drive. This procedure was not only time consuming but was also prone to human operator errors. Automated storage libraries such as that disclosed in U.S. Pat. No. 5,015,139 have been developed to overcome the delays associated with loading and unloading tapes. In addition, automated storage libraries eliminate the above described human operator errors associated with handling the tapes.
It is now possible, through the use of automated storage libraries, to automatically retrieve a data storage element (e.g. a tape cassette) from a storage location, transport it to a drive unit associated with the computer system and insert it therein. In this manner, human operator error can be avoided and request to load time can be significantly decreased.
The ability to place larger amounts of data on tape has derived principally from two technological advancements. First, actual cartridge size has been diminished as a result of various mechanical advances. Secondly, media properties and associated high density MR recording heads have been improved and developed to store more data on a given area of tape. Data can currently be written to 36 tracks or more. It can be thus be seen that with the improvements in physical size, thin film magnetic head and media technologies that have taken place over the years, it has become possible to pack more and more data into a smaller and smaller storage element such a tape cartridge. These advances in magnetic tape and head technology have made automated tape libraries more appealing in recent years.
Access time and reliability is improved in an automated tape library by automatically managing the storage and retrieval of tape cartridges. Operational benefits of using an automated tape library include greater reliability in tape cartridge mounts, better predictability in request-to-mount time and improved off-shift availability. Automated tape libraries include a large number of storage slots for storing library resident tape cartridges as well as one or more tape drives connected to the data processing system. They also include a picker mechanism which operates on command from the processing system to transfer a tape cartridge between a storage slot and a tape drive within seconds. In some cases it may also move a cartridge from one storage slot to another.
The robotic picker typically includes a mechanical gripper for retrieving the tapes and a system for locating the proper tape to retrieve or store. The tape may be located via a vision system residing on the picker mechanism. This vision system can view fidicial marks or a bar code attached to a tape cartridge in order to identify the correct cartridge to be picked. The vision system may also provide feedback to the gripper system so that the gripper system can move to the correct position in order to pick the designated tape cartridge.
It will be understood by one of ordinary skill in the art that such a picker can be configured to access and transport a variety of data elements, tape cartridges and optical disk cartridges being only two such elements. As such, it will be realized that the novel aspects of this invention can be applied in any sort of automated storage library, the automated tape library being only one possible application.
While the above-described automated storage libraries provide many advantages, they also suffer from drawbacks which can limit their potential application. This is especially true as databases get larger and computer floor space is at a premium. The library systems of the prior art typically only store cartridges in two dimensional arrays. In the case of a linear library such as the IBM 3495 Tape Library Dataserver, it is the height and length of the library and in the case of a circular library such as the 4400 Automated Cartridge System manufactured by the Storage Technology Corp., it is the circumference and the height.
The space aspect ratio for a linear library resulting from a two dimensional array could be as high as 18 to 1. In other words, the length of the library can be up to 18 times that of its width in order to hold the number of data cartridges required for the application. With space aspect ratios this high, much of an installation's usable floor space can easily be taken up by tape cartridge storage. Moreover, some users simply can not tolerate the spatial demands imposed by tape libraries in the prior art configuration.
The size of the picker mechanism can also be a limiting factor. Mechanically operated picker mechanisms can be large and complex. This is due to the requirement for servo mechanisms and the mechanical controls which are not required with a magnetic picker mechanism.
It is also important that the cartridge cells in the library be spaced sufficiently apart to allow the picker assembly to grab the cartridge without interference from other cartridges or portions of the cell enclosure. This, as a result, reduces cartridge packing density in the automated storage library and requires a larger storage area to contain the same amount of data.
There are various other constraints on the design of the cartridge storage area. For example, the cartridge in a library storage cell often must be tilted backwards to prevent it from falling out during environmental events such as earthquakes. In some cases, the possibility and severity potential of earthquake excitation is so high that an additional locking mechanism must be employed to prevent tape cartridges from falling out of their storage locations.
It should be noted that although the theoretical maximum storage density limit for IBM 3490 form factor cartridges (using a very narrow picker mechanism) is about 165 cartridges/sq. ft., the current state of the art maximum storage density for the same cartridge type in an automated storage library is about 59 cartridges/sq. ft. It can thus be seen that, with proper design and innovation, cartridge densities closer to the theoretical limit can be achieved.